This invention relates to a method and apparatus for measuring the period of an oscillatory signal burst containing only a limited number of cycles and occurring at a time which is random (and not known a priori).
An example of such a random signal occurs in making measurements of velocity using the Doppler shift in frequency of laser light scattered by a moving particle by heterodyning the scattered light with the unshifted light to produce a beat frequency burst at the optical receiver of the system. The phenomenon of Doppler shift in frequency is also utilized by many other systems that measure frequency to determine the velocity of a target, such as sonar and radar.
To appreciate the utility of this invention, consider how a traditional frequency counter measures frequency. A traditional frequency counter opens a gate G for a fixed length of time T (say one second) and counts the number of level crossings, of the same sense, of an input signal occurring during the time interval the gate is open (high). (See FIG. 1) This count, N, is then displayed as the frequency of the input signal in cycles per second (Hertz).
It is easy to see that such an instrument is of no use unless it can be guaranteed that the input signal oscillations continue for the whole interval that the signal is accepted by the gate. This proviso is, of course, not met by an oscillatory signal that occurs in bursts, at random times and for a variable number of cycles since a burst may not last for the whole counting interval. This problem is illustrated in FIG. 2 using the same gate of fixed time T.
It is the purpose of this invention to provide the means of measuring the frequency (period) of an input signal consisting of a single burst of an unknown number of cycles of a typical form shown in FIG. 2. A preferred embodiment of the invention was developed to provide the means of analyzing a laser Doppler velocimeter signal in the time domain. Consequently, the motivation for the various different modes of operation, the built-in flexibility of the first implementation of the idea and the available range of values of its operating characteristics (pulse widths, time intervals, etc.) are to be understood with that specific application in mind. It should be emphasized, however, that the utility of this signal processing method extends considerably beyond this purpose. The examples of sonar and radar have already been mentioned. Similar signals are also encountered in earthquake monitoring, where the time (duration) of the signal burst is also not known a priori, and in communications utilizing bursts of frequency modulated or pulse code modulated signals whose carrier frequency for one reason or another cannot be transmitted continuously.
The processing electronics of the invention should be viewed generally as possessing sufficient pattern recognition capabilities to be able to recognize the type of signal burst described above and respond to it for the purposes of measuring its average frequency and/or the instantaneous period on a cycle-by-cycle basis, and the real time of the event. It is for this reason that the processor that incorporates these features, a detailed description of which follows, is denominated a signal responsive burst period counter and timer, hereafter referred to simply as the system.
To achieve this goal, both the amplitude information and the time between successive crossings of zero, or some other predetermined level, are utilized in order to minimize the possibility that random noise will trigger the system and yield a reading in the absence of a valid signal burst. In fact this latter feature renders this device the ideal means to label an event in time electronically, which must be detected elsewhere, for which the penalty of error (false alarm) is very high. For example, one could code the event by a succession of pulses of a predetermined number and period, e.g., 32 pulses spaced individually 13.7.+-.0.1 .mu.sec apart. As will become apparent in the detailed description that follows, the system can be operated in a mode that will not produce a response even if any of the following, almost similar, events have occurred:
i. 31 pulses, or less, individually spaced by 13.7.+-.0.1 .mu.sec apart. PA1 ii. 33 pulses, or more, individually spaced by 13.7.+-.0.1 .mu.sec apart. PA1 iii. 32 pulses exactly, spaced 13.7.+-.0.1 .mu.sec apart on the average but having at least one pair spaced by a smaller or larger interval, with another pair, somewhere else in the pulse train that is appropriately spaced such that the average spacing is within the specified limits.
In order to introduce the motivating principles for the invention, a brief description of laser Doppler velocimetry fundamentals will be presented. There are two common optical arrangements used when making laser Doppler velocimetry measurements. One is the dual-scatter arrangement shown in FIG. 3 in which a laser beam 10 is split by a beam splitter 11 into two parallel beams 10a and 10b of equal intensity. A single focusing lens 12 will focus both beams and will force the two beams to cross (overlap) at the focus, F. The beams have finite beam widths and planar face fronts. Consequently, as the beams go through the overlap volume, the two beams can be considered as plane waves that can form an interference pattern on the surface of a square law detector 13, with linear fringe spacing, s, given by the equation EQU s = .lambda./2sin(.theta./2)
where .theta. is the angle subtended by the two wave vectors that are normal to the planar phase fronts.
If a particle traverses the overlap volume, it will scatter light in all directions and, in particular, in the directions accepted by collection optics 14-16 and narrow band optical filter in front of the detector, a photomultiplier. There the Doppler shifted amplitudes that have been scattered out of the two beams will interfere to yield a photocurrent which is modulated at a frequency, .nu..sub.D, given by the equation EQU .nu..sub.D = v.sub.195 /s = 2v.sub.195 /.lambda. sin (.theta./2)
where v.sub.195 is the component of the velocity in the plane of the two beams and perpendicular to their bisector. Since the distance travelled by the scattered light from the two beams is the same between the scattering particle and the photomultiplier, the phase difference of the heterodyning components can be computed equally well on the position of the scattering particle as it traverses the overlap volume. It is therefore equivalent to imagine that the particle is traversing a spatially modulated intensity field and that the photomultiplier sees a temporally modulated intensity whose frequency is given by the particle's velocity component perpendicular to the fringe planes, divided by the fringe spacing. It is in this sense that Eq. 2 is derived. In addition to the modulation at the Doppler frequency, the photocurrent is further characterized by a Gaussian envelope corresponding to the intensity distribution of the overlapping beams. The particular photocurrent patterns that result depend on the way the scattering particle has traversed the overlap volume. Three typical patterns are shown in FIGS. 4a-4c.
In the second optical arrangement, a reference-scatter arrangement, the laser beam 10 is again split into two parallel beams as shown in FIG. 5. However, in this case, one beam, the scattering beam 10a, has most of the laser power, while the other, the reference beam 10b, has a small fraction of the laser power. The two beams are focused on a common point, F, and meet in an overlap volume with approximately planar phase fronts. A scattering particle moving through this overlap volume scatters light from the scattering beam in all directions and, in particular, in the direction of the reference beam which is assumed to be normally incident on the surface of the photomultiplier 13. The photomultiplier thus sees two waves, the reference beam at the laser frequency .nu..sub.o, and the scattered wave at a frequency .nu..sub.o +.nu..sub.D. The Doppler shift .nu..sub.D is given by Eq. (2) where .theta. is the scattering angle (and also the angle subtended by the two beams). Thus the photocurrent, which is proportional to the incident intensity, is modulated at the beat frequency .nu..sub.D (between the two waves) for a time corresponding to the time of passage of a scattering particle through the overlap volume. Typical output patterns in this operating mode are shown in FIGS. 6a-6c.
In either case, if one can determine .nu..sub.D, the modulation frequency within the burst, one can measure (to the same accuracy) the component of the velocity of the scattering particle that is in the plane of the two beams and perpendicular to their bisector (perpendicular to the fringe planes). This can result in a complete vector velocity measurement since, to measure a different component, one can simply rotate the beams as required. By this means one actually measures the velocity of the fluid in the common case where the particles move with the local fluid velocity. This can be achieved by processing as many input channels as necessary to achieve a simultaneous measurement of more than one component of the velocity at one or more locations in the flow.